A digital imaging system typically includes a lens for focusing an object image on an image sensor, such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) imaging device. The image sensor includes an array of monochromatic photo detectors aligned in rows along the x-direction and columns along the y-direction. Each photo detector in the array of photo detectors corresponds to one picture element, referred to as a pixel, in a graphic image. The lens directs light from the object image onto the photo detectors of the image sensor to excite the photo detectors. To capture an object image, the photo detectors are sampled by the imaging system electronics. The sampled data is processed to produce an image.
To capture a color image of the object image, a color filter array (CFA) is placed over the photo detectors of the image sensor. The CFA includes filters aligned in rows along the x-direction and columns along the y-direction. Each filter in the CFA is aligned over one photo detector of the image sensor. The CFA filters light directed from the lens toward the photo detectors. The filtered light strikes the photo detectors of the image sensor and the photo detectors are sampled by the imaging system electronics. The sampled data is processed to produce a color image.
Filters in the CFA are typically organized in a mosaic pattern that resembles a three color checkerboard. In one type of CFA, the three colors in the mosaic pattern are the three primary colors of red, green and blue. As a result, each pixel captures the intensity of one of the primary colors of red, green or blue. In another type of CFA, the three colors in the mosaic pattern are the secondary colors of magenta, yellow, and cyan. As a result, each pixel captures the intensity of one of the secondary colors of magenta, yellow, or cyan.
Imaging systems with three color mosaic based image sensors have inherent drawbacks. No matter how many pixels a three color mosaic based image sensor contains, the image sensor can only capture one color at each pixel. Imaging systems with three color mosaic based image sensors rely on complex processing to interpolate the remaining two-thirds of the colors that are not captured. This interpolation leads to a loss of color resolution and color artifacts. To compensate, some imaging systems intentionally blur images to reduce artifacts.
Some applications require more color resolution than that provided by a three color mosaic based image sensor. In applications, such as distinguishing color variations in a succession of objects or tracking color changes in a scene over time, three colors may not provide enough color resolution. These applications include determining the color of products on a conveyor belt, determining color differences in food produce, and control of ambient lighting. Also, three colors may not provide enough color resolution in applications such as color process control where color measurements are taken and quality control is an issue.
For these and other reasons there is a need for the present invention.